Exoplanet Secondary Atmosphere Loss and Revival

Exoplanet secondary atmosphere loss and revival Edwin S. Kite1,* and Megan Barnett1. 1. University of Chicago, Chicago, IL. *Corresponding author. [email protected] Abstract. The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus – high-molecular-weight (secondary) atmospheres – on rocky exoplanets. Many rocky exoplanets are born with thick (> 10 kbar) H2-dominated atmospheres but subsequently lose their H2; this process has no known Solar System analog. We study the consequences of early loss of a thick H2 atmosphere for subsequent occurrence of a high- molecular-weight atmosphere using a simple model of atmosphere evolution (including atmosphere loss to space, magma ocean crystallization, and volcanic outgassing). We also calculate atmosphere survival for rocky worlds that start with no H2. Our results imply that most rocky exoplanets orbiting closer to their star than the Habitable Zone that were formed with thick H2-dominated atmospheres lack high-molecular-weight atmospheres today. During early magma ocean crystallization, high-molecular-weight species usually do not form long-lived high-molecular-weight atmospheres; instead they are lost to space alongside H2. This early volatile depletion also makes it more difficult for later volcanic outgassing to revive the atmosphere. However, atmospheres should persist on worlds that start with abundant volatiles (for example, waterworlds). Our results imply that in order to find high–molecular-weight atmospheres on warm exoplanets orbiting M-stars, we should target worlds that formed H2-poor, that have anomalously large radii, or that orbit less active stars. Significance statement. Earth and Venus have significant atmospheres, but Mercury does not. Thousands of exoplanets are known, but we know almost nothing about the rocky exoplanet atmospheres. Many rocky exoplanets were formed by a sub-Neptune to super-Earth conversion process during which planets lose most of their H2-rich (primary) atmospheres and are reduced in volume by a factor of >2. Does such a gas-rich adolescence increase or decrease the likelihood that super-Earths will subsequently exhibit a H2-poor (secondary) atmosphere? We show that secondary atmospheres exsolved from the magma ocean are unlikely to be retained by super-Earths, but it is possible for volcanic outgassing to revive super-Earth atmospheres. For M-dwarf planetary systems, super-Earths that have atmospheres close to the star likely were formed with abundant volatiles. Introduction. The Solar System has three planets – Earth, Venus, and Mars – that have atmospheres derived from H2-free solids (secondary atmospheres), and four giant planets whose atmospheres are derived from protoplanetary nebula gas (H2-dominated primary 1 atmospheres) (Atreya et al. 1989, Marty et al. 2016). However, this clean separation in process and outcome is apparently unrepresentative of the known exoplanets. The two most common types of known exoplanet, the rocky super-Earth-sized planets (planet radius Rpl < 1.6 R⊕, where “⊕” is the Earth symbol, planet density ρpl > 4 g/cc; “super-Earths”) and the gas-shrouded sub-Neptunes (Rpl = 2 - 3 R⊕), are divided by a valley in (planet radius)-(orbital period) space in which planets are less common (Fig. 1) (Fulton et al. 2017). The radius valley can be understood if, and only if, a substantial fraction of planets that are born with thick (>10 kbar) H2-dominated primary atmospheres lose those atmospheres and shrink in radius to become rocky super-Earths (Van Eylen et al. 2018). This radius-shrinking process, which carves out the radius valley, may be the way that most super-Earths form. There is no evidence that Earth and Venus underwent this process, and primary atmospheres are not thought to have contributed to the origin of major volatile elements in Earth (Dauphas & Morbidelli 2014). Both Venus and Earth have secondary volatile envelopes (composed of solid-derived volatiles, including H2O and CO2) that are much less massive than the atmospheres of sub-Neptunes. Large surface reservoirs of H2O, C species, and N species are essential to life on Earth and to Earth’s habitable climate (Bergin et al. 2015, Catling & Kasting 2017). Does forming with a thick primary atmosphere (sub-Neptune) but ending up as a rocky super-Earth favor the rocky planet ending up with a secondary atmosphere? Do primary atmospheres, in dying, shield high-molecular-weight species from loss during the early era of atmosphere-stripping impacts and intense stellar activity, allowing those constituents to later form a secondary atmosphere? Or does the light (H2-dominated) and transient primary atmosphere drag away the higher-molecular weight species? Getting physical insight into the transition from primary to secondary atmospheres is particularly important for rocky exoplanets that are too hot for life. Hot rocky exoplanets are the highest signal/noise rocky targets for upcoming missions such as James Webb Space Telescope (JWST) (Kempton et al. 2018) and so will be the most useful for checking our understanding of this atmospheric transition process. Secondary atmospheres are central to the exoplanet exploration strategy (National Academy of Sciences 2018, Zahnle & Catling 2017). Previous work on the hypothesis that primary atmospheres played a role in forming secondary atmospheres includes that of Eucken in the 1940s (Turekian & Clark 1969), Urey (1951), Cameron and coworkers (e.g. Slattery et al. 1980), Sasaki (1990), and Ozima & Zahnle (1993). Secondary atmosphere formation on exoplanets in the absence of a primary atmosphere has been investigated by (e.g.) Elkins-Tanton & Seager (2008), Dorn et al. (2018), and references therein. Gas survival during planetary volume reduction. During sub-Neptune to super-Earth conversion, we suppose the planet contains both nebular-derived H2 and also high-μ species (derived from the planet-forming solid materials) that could form a secondary atmosphere – if retained. Retention of volatiles is controlled by atmospheric loss and atmosphere-interior exchange (Fig. 2). For both sub- 2 Neptunes and super-Earths, silicates (both magma and solid rock) apparently make up most of the planet’s mass (e.g. Dai et al. 2019, Owen and Wu 2017). Atmosphere loss is more difficult for high-molecular-weight volatiles than for H2 because higher-molecular-weight volatiles such as H2O are more easily shielded within the silicate interior and are also more resistant to escape. This can be understood as follows. First, a basic control on atmosphere loss is the ratio, λ, of gravitational binding energy to thermal energy: λ = (μ / k Tua) (G Mpl / (R + z ) ) 4 ≈ 2.3 μ (10 K / Tua)( Mpl / 6 M⊕)/( (R + z ) / 2 R⊕) (1) where μ is molecular mass, k is Boltzmann’s constant, Tua is upper-atmosphere temperature, G is the gravitational constant, Mpl is planet mass, R is the radius of the silicate planet, and z is atmosphere thickness. When λ ≲ 2, the upper atmosphere flows out to space at a rate potentially limited only by the energy available from upper-atmosphere absorption of light from the star (Watson et al. 1981). If the upper atmosphere absorbs 100 W/m2 of light from the star, the upper limit on loss is ~1000 bars/Myr. This hydrodynamic outflow ejects the tenuous upper atmosphere at high speed, but the dense lower atmosphere remains close to hydrostatic eQuilibrium. When λ > 10, the hydrodynamic outflow shuts down. For primary atmospheres, the mean molecular weight μavg ≈ 2 Da (H2) while for secondary atmospheres, μavg is ≳10× higher favoring secondary atmosphere retention. Moreover, many secondary-atmosphere constituents (e.g CO2) are much more effective coolants than H2, so for secondary atmospheres Tua is lower (e.g. Kulikov et al. 2007, Johnstone et al. 2018). Atmospheric thickness z is also smaller for high-μavg atmospheres due to their smaller scale height, raising λ. For all atmospheres, proximity to the star increases Tua (lowering λ) and also increases the upper atmosphere outflow rate when the λ ≲ 2 condition is satisfied. Moreover, closer to the star impacts occur at higher velocities that are more erosive (Kegerreis et al. 2020). Thus we expect massive worlds far from the star to retain atmospheres and low-mass worlds closer to the star to lose them (Zahnle & Catling 2017). This expectation, while physically valid, offers little guidance as to whether or not super-Earths will have atmospheres. To go further, we need to consider the effect of evolving atmospheric composition on loss rate (EQn. 1) and also track the shielding of volatiles within silicates (magma oceans and solid rock) (Fig. 2). The atmospheres of young sub-Neptunes are underlain by magma oceans (e.g. Vazan et al. 2018; Fig. S1). EQuilibrium partitioning of a volatile i between the atmosphere (where it is vulnerable to escape) and the magma ocean (where it is shielded from escape) is given by the following: ci = ei + pi si mmagma + other reservoirs = ei + [ (ei / Amai) gmai (μavg/ μi ) ] si mmagma + other reservoirs (2) where ci is the total inventory of the volatile, ei is the mass in the atmosphere, pi is the partial pressure of i at the magma-atmosphere interface, Amai (and gmai) are the area of (and 3 gravitational acceleration at) the magma-atmosphere interface, μavg is the mean μ of the -1 atmosphere, si is the solubility coefficient (mass fraction pascal ) of the volatile in the magma, and mmagma is the mass of liQuid in the magma ocean. Solubility in magma is higher for many secondary-atmosphere constituents (e.g. CH4, H2S, H2O) than for H2. When the atmosphere contains both H2 and a more-soluble, high-μ constituent, the H2 takes the brunt of atmosphere loss processes that might otherwise remove the high-μ species. Differential solubility also protects the more-soluble species from impact shock (Genda & Abe 2005). Other reservoirs include volatiles stored in the crystal phase. This is an unimportant reservoir early on when the planet has a massive magma ocean, but becomes important later, because these volatiles can be released by volcanic degassing.

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